1. Field of the Invention
The invention relates to a semiconductor integrated circuit including a copper wiring layer, and more particularly to a barrier film which prevents copper diffusion from such a copper wiring layer.
2. Description of the Related Art
As a semiconductor device has been designed to be smaller and smaller in size, wiring delay exerts greater influence on a silicon ULSI device. As a result, though a wiring layer has been composed of aluminum, it is necessary to compose a wiring layer of copper in place of aluminum.
Resistivity of copper is equal to about 70% of resistivity of aluminum. However, since copper does not form passive state composed of an oxide film, at a surface thereof, unlike aluminum, copper is more corrosive than aluminum.
In addition, since copper has a high diffusion rate in both silicon (Si) and silicon dioxide (SiO2), if copper enters MOSFET formed on a silicon substrate, copper would induce reduction in carrier lifetime.
Hence, it is absolutely necessary for a semiconductor device having a copper wiring layer to have a diffusion-barrier film for preventing diffusion of copper into an interlayer insulating film formed between copper wiring layers. In addition, since such a diffusion-barrier film has to have high adhesion characteristic to both an interlayer insulating film and a copper wiring layer in order to keep reliability in wiring.
Thus, there have been made many suggestions about a structure of a barrier metal layer and a method of fabricating the same, in order to prevent copper diffusion form a copper wiring layer.
For instance, a structure of a barrier metal layer is suggested in the following articles:
(a) Semiconductor World, Nobuyoshi Awaya, February 1998, pp. 91-96 (hereinafter, referred to as Prior Art 1);
(b) Advanced Metallization and Interconnect Systems for ULSI Applications in 1997, Kee-Won Kwon et al., 1998, pp. 711-716 (hereinafter, referred to Prior Art 2);
(c) Journal Electrochemical Society, M. T. Wang et al., July 1998, pp. 2538-2545 (hereinafter, referred to as Prior Art 3); and
(d) 1998 Symposium on VLSI Technology Digest of Technical Papers, D. Denning et al., 1998, pp. 22-23.
In addition, a structure of a barrier metal layer and a method of fabricating the same both for preventing copper diffusion is suggested also in Japanese Unexamined Patent Publications 8-139092, 8-274098, 9-64044 and 10-256256, and Japanese Patent Application No. 10-330938. Herein, Japanese Patent Application No. 10-330938 is not published yet, and hence does not constitute prior art to the present invention. However, it is explained in the specification only for better understanding of the present invention. The applicant does not admit that Japanese Patent Application No. 10-330938 constitutes prior art to the present invention.
It is quite difficult to dry-etch copper, and hence, a copper wiring layer is formed generally by chemical mechanical polishing (CMP).
Specifically, a copper wiring layer is formed as follows.
An insulating film is formed on an underlying copper wiring layer. Then, the insulating film is formed with a recess and a through-hole reaching the underlying copper wiring layer. Then, a thin diffusion-barrier film is formed on surfaces of the recess and the through-hole therewith such that the recess and the through-hole is completely covered at surfaces thereof with the diffusion-barrier film in order to prevent copper diffusion from uncovered region.
Thereafter, a copper film is deposited filling the recess and the through-hole therewith by CVD or sputtering. Then, the copper film and the diffusion-barrier film are removed in selected regions by CMP. Thus, a copper wiring layer is completed.
As will be obvious to those skilled in the art, the diffusion-barrier film is required to have high coverage as well as capability of preventing copper diffusion and adhesion to copper.
The diffusion-barrier film is composed, for instance, of refractive metal such as tungsten (W), tantalum (Ta) or titanium (Ti), or nitride of such refractive metal such as tungsten nitride (WN), titanium nitride (TiN) or tantalum nitride (TaN).
As explained in Prior Art 2, for instance, a tantalum (Ta) barrier film has high adhesion with a copper film formed on the tantalum barrier film by sputtering, ensuring improvement in crystallinity of the copper film. However, since copper is diffused into the tantalum film, it would be necessary for the tantalum barrier film formed below the copper film, to have a thickness of 50 nm or greater.
Prior Art 4 reports that if a copper film is formed on a tantalum film by CVD, fluorine (F) segregates at an interface between the copper film and TaN, resulting in degradation in adhesion therebetween.
Prior Art 3 reports that a crystalline TaN barrier film oriented in directions of (200) and (111) can prevent copper diffusion more highly than a crystalline Ta barrier film.
As an solution to enhance a characteristic of preventing copper diffusion and adhesion to copper, a multi-layered structure of a metal film and a metal nitride film has been suggested.
For instance, the above-mentioned Japanese Patent Application No. 10-330938 has suggested a method of fabricating a multi-layered barrier film including a titanium film and formed by sputtering.
As illustrated in FIG. 1, in accordance with the suggested method, only an argon gas is introduced into a sputter chamber to thereby form a titanium film 1. Then, a nitrogen gas is introduced into the sputter chamber, and a thin titanium nitride film 2 is formed on the titanium film 1 auxiliarily making use of reaction between titanium and nitrogen. Thus, there is formed a multi-layered barrier structure 3 comprised of the titanium film 1 and the thin titanium nitride film 2.
In the method, a metal oxide film formed on an underlying wiring film is removed by argon plasma prior to carrying out sputtering.
However, the conventional barrier film for preventing copper diffusion is accompanied with the following problems.
The first problem is that it is quite difficult to make a diffusion-barrier film have both a characteristic of preventing copper diffusion and a sufficient adhesive force with copper.
As illustrated in FIG. 2, it is now assumed to form a metal film 5 having a crystallized pillar structure, on a semiconductor substrate 4. In the metal film 5, a lot of grains each comprised of individual crystals, and grain boundaries 7 each defining an interface between the grains 6 exist throughout the metal film 5, that is, from an upper surface to a bottom of the metal film 5. The grain boundaries 7 define paths 8 through which copper is diffused. As a result, the metal film 5 has low barrier characteristic of preventing copper diffusion.
As illustrated in FIG. 3, it is now assumed to form a metal film 5a on a semiconductor substrate 4. If the metal film 5a is composed of metals having small resistivity, such as tungsten (W), titanium (Ti) or tantalum (Ta), the metal film 5 would have a polycrystal structure. As a result, the metal film 5a would have a pillar-like structure similarly to the metal film 5 illustrated in FIG. 2, and accordingly, the metal film 5a would have small barrier characteristic of preventing copper diffusion.
However, it should be noted that if a copper film is formed on a crystalline metal film, such as a xcex2-Ta (002) film as obtained in sputtering of a tantalum film, by sputtering, the copper film would have high adhesion and rich crystal orientation, though a barrier characteristic of preventing copper diffusion would be deteriorated. As a result, the copper film would enhance reliability in copper wiring.
In contrast, the metal film 5a illustrated in FIG. 3, which is composed of particles 9 such as amorphous TaN and formed on the semiconductor substrate 4, has small resistivity, specifically in the range of about 200 to 250 xcexcxcexa9cm, and does not have the paths through which copper is diffused unlike the crystalline metal film 5 illustrated in FIG. 2. As a result, the metal film 5a would have high barrier characteristic of preventing copper diffusion.
However, since a surface of the metal film 5a is amorphous and hence crystal lattice is not uniformly arranged, if a copper film s formed on the amorphous metal film 5a by CVD or sputtering, copper crystallinity and adhesion to copper are degraded.
As mentioned so far, it is quite difficult to form a diffusion-barrier film having a single-layered structure comprised only of a crystalline metal film or an amorphous metal nitride film, and further having high barrier characteristic of preventing copper diffusion and high adhesion to copper.
The second problem is caused when a diffusion-barrier film is designed to have a multi-layered structure in order to avoid the above-mentioned problem of the single-layered diffusion-barrier film.
For instance, if a diffusion-barrier film is designed to have a multi-layered structure comprised of a crystalline metal film having high adhesion to copper and an amorphous metal nitride film having high barrier characteristic, such as TaN, there would be obtained a diffusion-barrier film having high barrier characteristic of preventing copper diffusion and high adhesion to copper.
However, since it was not possible in a conventional method to successively form a crystalline metal film and an amorphous metal nitride film by sputtering, the crystalline metal film and the amorphous metal nitride film had to be separately formed in the same sputtering chamber or be formed in separate sputtering chambers.
For instance, the above-mentioned Japanese Patent Application No. 10-330938 has suggested a method including the steps of introducing an argon gas into a sputtering chamber to thereby form a titanium film, and introducing a nitrogen gas into the sputtering chamber to thereby form a titanium nitride film on the titanium film.
However, in accordance with this method, the titanium nitride film cannot be formed until partial pressures of argon and nitrogen become stable by varying a mixture ratio of argon and nitrogen. Hence, it is impossible to enhance a fabrication yield of fabricating a diffusion-barrier film having a multi-layered structure.
The third problem relates to coverage of a film formed by sputtering.
In general, when a metal film or a metal nitride film is formed by sputtering, a metal target is sputtered by argon plasma generated by virtue of rotational magnetic field and application of DC bias, and resultingly, a metal film or a metal nitride film is deposited on a substrate located in facing relation to the metal target.
In sputtering, a pressure at which a metal target is sputtered is low, specifically, equal to 1 Pa or smaller. Since metal particles sputtered by argon plasma are radiated randomly to a surface of a substrate, for instance, if the substrate is formed at a surface thereof with a deep recess or hole, it would almost impossible to deposit a metal film such that such a recess or hole is completely covered with the metal film.
In addition, since a sputtering pressure is low, argon plasma could have a low plasma density, and hence, there cannot be expected re-sputtering effect in which a metal film deposited onto a surface of a substrate is sputtered by argon plasma.
In order to enhance coverage of a metal film, there has been suggested collimate sputtering in which a metal plate formed with a lot of through-holes is located between a sputtering target and a substrate, and metal particles are caused to pass through the through-holes to thereby uniform direction of metal particles. In accordance with the collimate sputtering, it is possible to deposit a metal film on a bottom of a recess formed at a surface of a substrate, but it is not possible to deposit a metal film onto an inner sidewall of the recess.
The fourth problem is that a crystalline metal film having high adhesion with a copper film tends to react with atmosphere to thereby a reaction layer at a surface thereof.
Such a reaction layer would much deteriorate adhesion of a metal film with a copper film.
The fifth problem is a copper oxide film is adhered again to a recess or hole.
An oxide film formed on a surface of an underlying wiring metal film is removed by argon plasma prior to deposition of a diffusion-barrier film by sputtering. When an underlying wiring layer is composed of copper, a copper oxide film is scattered by argon sputtering, and as a result, the thus scattered copper oxide is adhered again to a recess or hole formed at a surface of an insulating film.
The sixth problem is that when a copper film is formed on a tantalum film and an amorphous TaN film by CVD, adhesion between the copper film and a diffusion-barrier film is deteriorated.
In view of the above-mentioned problems in a conventional diffusion-barrier film, it is an object of the present invention to provide a diffusion-barrier film having both a diffusion-barrier characteristic of preventing copper from being diffused into a semiconductor device and high adhesion between a copper film and an interlayer insulating film.
It is also an object of the present invention to provide a multi-layered wiring structure including the above-mentioned diffusion-barrier film.
Another object of the present invention is to provide a method of fabricating such the above-mentioned diffusion-barrier film.
A further object of the present invention is to provide a method of fabricating a multi-layered copper wiring layer in which copper is buried above the above-mentioned diffusion-barrier film.
In one aspect of the present invention, there is provided a barrier film preventing diffusion of copper from a copper wiring layer formed on a semiconductor substrate, including a multi-layered structure of first and second films, the first film being composed of crystalline metal containing nitrogen therein, the second film being composed of amorphous metal nitride, the barrier film being constituted of common metal atomic species.
It is preferable that the first film is formed on the second film.
It is preferable that the second film has a thickness in the range of 80 angstroms to 150 angstroms both inclusive.
It is preferable that the first film has a thickness in the range of 60 angstroms to 300 angstroms both inclusive.
In another aspect of the present invention, there is provided a multi-layered wiring structure including a barrier film which prevents diffusion of copper from a copper wiring layer formed on a semiconductor substrate, the barrier film having a multi-layered structure of first and second films, the first film being composed of crystalline metal containing nitrogen therein, the second film being composed of amorphous metal nitride, the barrier film being constituted of common metal atomic species.
It is preferable that the barrier film covers a recess and a hole formed throughout an insulating film formed on an underlying wiring layer.
It is preferable that the multi-layered wiring structure further includes a copper film formed on the first film.
In still another aspect of the present invention, there is provided a method of forming a diffusion-barrier film by sputtering, including the steps of (a) preparing gas containing nitrogen therein, and (b) varying only power of an electric power source for generating plasma to thereby successively form a diffusion-barrier film having a multi-layered structure of first and second films, the first film being composed of crystalline metal containing nitrogen therein, the second film being composed of amorphous metal nitride, the barrier film being constituted of metal atomic species of sputter target.
It is preferable that the gas containing nitrogen therein has a pressure equal to or greater than 5 Pa.
It is preferable that the gas contains nitrogen at 10 volume % or smaller.
It is preferable that the metal atomic species of sputter target is one of tantalum, tungsten, titanium, molybdenum and niobium alone or in combination.
It is preferable that the second film has a thickness in the range of 80 angstroms to 150 angstroms both inclusive.
It is preferable that the first film has a thickness in the range of 60 angstroms to 300 angstroms both inclusive.
There is further provided a method of forming a diffusion-barrier film by RF magnetron sputtering making use of rotational magnetic field and RF power, including the steps of (a) preparing gas containing nitrogen therein, and (b) varying the RF power to thereby successively form a diffusion-barrier film having a multi-layered structure of first and second films, the first film being composed of crystalline metal containing nitrogen therein, the second film being composed of amorphous metal nitride, the barrier film being constituted of metal atomic species of sputter target.
There is still further provided a method of forming a diffusion-barrier film by RF magnetron sputtering, including the steps of (a) setting an electric power source for generation plasma to generate power having a first value, to thereby a first film, with a concentration of nitrogen in plasma gas being kept at a constant, and (b) setting the electric power source to generate power having a second value greater than the first value at the moment when the first film is formed by a predetermined thickness, to thereby form a second film on the first film.
It is preferable that the first film is composed of amorphous metal nitride, and the second film is composed of crystalline metal containing nitrogen therein.
There is yet further provided a method of forming a copper wiring film, including the steps of (a) radiating plasma of argon containing hydrogen therein, to a recess or hole formed at an insulating film formed on a semiconductor substrate, (b) forming a diffusion-barrier film to cover the recess or hole therewith without exposing to atmosphere, the diffusion-barrier film having a multi-layered structure of first and second films, the first film being composed of crystalline metal containing nitrogen therein, the second film being composed of amorphous metal nitride, and (c) forming a copper film on the diffusion-barrier film without exposing to atmosphere.
It is preferable that the diffusion-barrier film is formed by sputtering.
It is preferable that the copper film is formed in vacuum.
It is preferable that the copper film is formed by thermal chemical vapor deposition in which thermal dismutation in a complex of organic metal is utilized.
It is preferable that the copper film is formed by sputtering in which copper target is used.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In the diffusion-barrier film in accordance with the present invention, a copper film makes direct contact with a crystalline metal film containing nitrogen therein, ensuring high adhesion therebetween and high crystallinity of a copper film.
In addition, since the metal film contains nitrogen therein, copper diffusion into a semiconductor device can be prevented more effectively than a metal film having pure crystals.
In the diffusion-barrier film in accordance with the present invention, an amorphous metal film containing nitrogen therein lies under a crystalline metal film containing nitrogen therein. Hence, it is possible to effectively prevent copper diffusion, and to ensure high adhesion with an underlying insulating film such as a silicon dioxide film. That is, by forming a copper wiring layer on the diffusion-barrier film in accordance with the present invention, it is possible to not only ensure high crystallinity and high adhesion of a copper wiring layer, but also to prevent copper diffusion.
The method in accordance with the present invention makes it possible to successively form a diffusion-barrier film having a multi-layered structure of first and second films, by varying only power of an electric power source for generating plasma in sputtering in which gas containing nitrogen therein is employed. Herein, the first film is composed of crystalline metal containing nitrogen therein, and the second film is composed of amorphous metal nitride. The barrier film is constituted of metal atomic species of sputter target.
Specifically, an electric power source for generating plasma is first set to generate relatively low power with a concentration of nitrogen in plasma gas being kept constant. A film is formed in such a condition. Target metal makes sufficient reaction with nitrogen, and resultingly, an amorphous metal nitride film is formed. Immediately after the formation of the amorphous metal nitride film, the electric power source is set to generate relatively high power to thereby form a film without allowing sufficient time for reaction between nitrogen and target metal. As a result, there is obtained a crystalline metal film containing nitrogen therein.
Thus, it is possible to successively form a diffusion-barrier film in the same chamber, wherein the diffusion-barrier film has a multi-layered structure including a crystalline metal film containing nitrogen therein and an amorphous metal nitride film.
The method of fabricating a diffusion-barrier film employs RF magnetron sputtering in which rotational magnetic field and RF power are utilized. Since the method makes it possible to carry out sputtering where a nitrogen-containing gas has a pressure equal to or greater than 5 Pa, plasma density of argon which is a main constituent of sputtering gas can be enhanced, and thus, there can be obtained coverage for entirely covering a recess or hole formed at a surface of a substrate, with the diffusion-barrier film.
The method of fabricating a diffusion-barrier film, in accordance with the present invention, includes the step of radiating plasma of argon containing hydrogen therein, to a recess or hole formed at an insulating film formed on a semiconductor substrate. This step reduces a copper oxide film formed on a surface of an underlying copper wiring layer, to thereby turn copper oxide back to copper, ensuring remarkable reduction in re-sputtering of a copper oxide film to a surface of a recess or hole formed at a surface of an insulating film.
Then, a diffusion-barrier film is formed to cover the recess or hole therewith without exposing to atmosphere, wherein the diffusion-barrier film has a multi-layered structure of first and second films, the first film being composed of crystalline metal containing nitrogen therein, the second film being composed of amorphous metal nitride. Then, a thin copper film is formed on the diffusion-barrier film in vacuum. As a result, there is obtained a multi-layered structure comprised of the diffusion-barrier film and the copper wiring film without a metal oxide layer being sandwiched therebetween.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.